(1) Field of the Invention
The present invention relates to a dual phase-structured hot rolled high-tensile strength steel sheet and a method of producing the same. More particularly the present invention relates to an inexpensive dual phase-structured hot rolled high-tensile strength steel sheet having a low yield ratio, a high tensile strength of about 50-80 kg/mm.sup.2 and excellent formability due to the dual phase structure consisting of ferrite phase and a second phase, such as martensite (including remaining austenite) or the like, dispersed in the ferrite phase; and to a method of producing advantageously the high tensile strength steel sheet in a simple manner by relaxing effectively the restriction for controlling the cooling step of a hot rolled sheet after hot rolling.
(2) Description of the Prior Art
There has recently been noticed a dual phase-structured steel sheet having a dual phase structure, which consists of a ferrite phase and a second phase dispersed therein, as a high-tensile strength steel sheet having excellent formability. This steel sheet is low in the yield strength (Y.S.) and high in the tensile strength (T.S.), and hence is low in the yield ratio (Y.R.) represented by (Y.S./T.S.).times.100, and is remarkably higher in the elongation (El.) than conventional steel sheets having the same T.S. as that of the dual phase-structured steel sheet. However, this characteristic properties do not appear in all ferritemartensite steels, but appears only when the fraction of ferrite phase is at least 70%, the fraction of a second phase is at least 5%, and further the fractions of pearlite and bainite. In this case, the steel has a low Y.R. of not higher than 70% and is excellent in the formability.
As a method of producing the dual phase-structured steel sheet, there have been known a method, wherein a hot rolled sheet is subjected to a continuous annealing and then cooled; and a method, wherein a hot rolled sheet is directly cooled without after-treatment. In the former method, as heat-treatment must be carried out, and the production cost of the steel sheet is high. Therefore, the latter method has been predominantly carried out recently.
Various methods have been proposed as a method of producing dual phase-structured steel sheet by cooling directly a hot rolled sheet, and these methods are generally classified into two methods. In one of the methods, a hot rolled sheet having a dual phase consisting of .alpha. and .gamma. phases is coiled as such, and the .gamma. phase is transformed into martensite during the cooling step after coiling. In another method, a ferrite.martensite microstructure is formed in a steel sheet during the cooling stage following to hot rolling, and then the steel sheet is coiled.
In the former method, a large amount of alloy element, such as Si, Mn, Cr, Mo and the like, must be added to steel in order to keep stably austenite until the martensite transformation occurs during the cooling step, and therefore the production cost of the dual phase-structured steel sheet is high. On the contrary, in the latter method, the addition amount of alloy elements, such as Si, Mn, Cr and the like, to steel can be decreased, but finishing rolling condition, cooling rate after rolling, cooling pattern and coiling temperature must be strictly controlled in order to obtain the above described ideal microstructure containing at least 70% of ferrite and at least 5% of a second phase. However, the latter method has still such drawback that, even when these conditions are strictly controlled, the mechanical property of the coiled steel is apt to be nonuniform in its length and width directions.
The inventors have investigated the above described drawbacks of conventional technics and made various experiments. As a result, the inventors have found that, in the case where a very inexpensive alloy element of P is used, even when the hot rolling condition is limited to a necessary but minimum condition, a dual phase-structured high-tensile strength steel sheet having a high ferrite fraction, a Y.R. of not higher than 70% and excellent ductility can be very inexpensively obtained by merely directly cooling a hot rolled sheet as such without any particular heat-treatment.
That is, in the latter method, the limitation of finishing rolling temperature and the use of a particular cooling pattern, which is carried out in the cooling step following to the finishing rolling and contains a slow cooling stage, have been considered to be indispensable conditions. For example, Japanese Patent Laid Open Application No. 91,934/80 discloses that a dual phase-structured steel sheet can not be obtained unless the finishing rolling is carried out at a low temperature and a hot rolled sheet is firstly cooled slowly and is then quenched. Contrary to this disclosure, the inventors have found that, when the above described steel contains at least 0.04% of P, even in the case where a finishing rolling is carried out at an ordinary finishing rolling temperature by means of a conventional continuous type hot mill and then the hot rolled sheet is cooled at a cooling rate within the ordinary cooling rate range (10.degree.-200.degree. C./sec), at least 70% of ferrite is formed and at least 5% of a second phase is uniformly dispersed in the ferrite due to the enrichment of C in austenite and to the action of Mn. The inventors have made further investigations and found that Si promotes ferrite transformation and enrichment of C in austenite to form martensite more easily, and Cr stabilizes the stability of austenite to increase the hardenability of martensite, whereby the tensile strength of the resulting hot rolled steel sheet is more increased.